Freescale Semiconductor, Inc.
Document Number: AN4866
Rev. 0, 01/2014
Application Note
Coils Used for Wireless Charging
1 Introduction
There are many battery-powered devices, which
need to be periodically charged, such as mobile
phones, tablets, and battery-powered hand tools.
These tools are usually charged by a dedicated
cord from a USB port or from a dedicated wall
adapter.
Wirelessly transferred power simplifies the
charging of these devices and brings more
convenience to the everyday lives of our
customers. Dedicated controllers are required for
wireless charging application control.
Contents
1
Introduction
1
2
Coil categories
2
3
Coil description and properties
3
4
A Multicoil solution
5
5
Conclusion
6
6
References
6
7
Revision history
6
Freescale provides the WCT1xxx family of the
dedicated controllers for the wireless charging
applications. Each type is intended for a certain
range of applications in the automotive or
industrial and consumer area. Freescale provides
the dedicated wireless charging software library
intended for these controllers. This library
maintains all functions required by the Qi
specification.
© 2014 Freescale Semiconductor, Inc.
___________________________________________________________________
2 Coil categories
Each application area requires a certain level of power transfer. The main aspects of the categorization are
as follows:

Transmitter or receiver side:
o The transmitter side generates the power electromagnetic field.
o The receiver side produces the AC/DC current when placed to the power electromagnetic
field.

A level of the transferred power:
o Low power – up to 5 W on the receiver side
o Medium power – from 5 W to 100 W
o High power – from 100 W to 1 kW level
o Very high power – more than 1 kW

Free-positioning:
o Single coil – with or without an attractor
o Multicoil solution

Manufacturing:
o Wire wound from the Litz wire
o PCB type of coils
2
Coils Used for Wireless Charging Application Note, Rev. 0, 01/2014
Freescale Semiconductor, Inc.
3 Coil description and properties
The requirements for the coil construction are as follows:

The inductance and power capability must meet the category requirements, including the working
frequency and transferring power.

The mechanical dimension must fit to the target application, including the coil area and thickness.

The coil must provide the electromagnetic shielding for the associated electronic device, such as
the mobile phone.

Durable construction.
Figure 1 shows the basic wire wound coil structure.
Figure 1 Wire wound coil structure (property of TDK)
The coil winding can be one or two layers of the Litz wire. The adhesive tape between coil and ferrite
sheet maintains the required distance and the mechanical fixation. The center positioned magnet can be
used to provide better alignment between the transmitter and the receiver coils. The ferrite sheet provides
the one side magnetic shield and forms the magnetic field. The ferrite properties (material and thickness)
have influence to the maximal power transfer for the certain temperature rise.
For the power level 5 W and higher, usually various types of the ferrite material are used. For the power
level lower than 5 W, the thin ferrite tape or other soft magnetic materials could be used. The thickness of
the magnetic shield is from 0.5 mm to 5 mm depending on the type of coil.
Coils Used for Wireless Charging Application Note, Rev. 0, 01/2014
Freescale Semiconductor, Inc.
3
A coil has the following properties:

Inductance ranging from 6 uH to 27 uH.

The DC resistance depends mainly on the used type of the Litz wire, usually ranging from 20 mΩ
to 100 mΩ.

The dimension depends on the required power transfer capability and the final application, usually
ranging from about 30 mm to 55 mm for the rectangular and the circular single coil.

The working frequency ranges from 105 kHz to 210 kHz for the devices which meet the Qi
specification.

The working voltage is from 5 V to 50 V in extreme conditions.
Figure 2 shows the examples of the wire wound coils.
Figure 2 Wire wound coils
Figure 3 shows the basic PCB coil structure.
Figure 3 Cross section of the PCB coils system
PCB coils consist of several layers of overlapped coils. All coils are independent and have their own
connection points. The coil shapes can vary from the rounded square to hexagonal or circular. The PCB
coils usually have higher internal DC resistance, lower quality, lower efficiency, lower thickness, and
lower cost. They can be combined with wire wound coils.
4
Coils Used for Wireless Charging Application Note, Rev. 0, 01/2014
Freescale Semiconductor, Inc.
4 A Multicoil solution
A multicoil solution is used when the free positioning design is required. The transmitter checks each coil
coupling to the receiver, and then selects the coil with the best coupling for the power transfer. It can
energize one coil only or several coils together. The simultaneously energized coils can provide higher
power transfer capability and larger freedom for the receiver coil placement.
The simultaneously energized coils are connected to the same AC current source with the same
electromagnetic field orientation. The connection can be maintained by the power multiplexer or the
separate power drivers can be used for each coil. This type of the power transfer, called as one channel
transfer, is intended for one receiver only. If you need to charge two or more devices simultaneously, you
need to use the multichannel topology. In this case, each channel works independently.
The example of the multicoil design is the Freescale Wireless Charger for automotive sector, topology
A13_Rev2. It is controlled by the WCT1001A controller. The final demo example is shown in Figure 4.
Figure 4 Multicoil design example
Figure 5 shows the block schematic of the power multiplexer.
Figure 5 Power multiplexer
The multiplexer is controlled by the wireless charging controller and it can connect one or more coils from
the three-coil set to the energy source. The power multiplexer can be spread to more segments to control
multicoil. Then one or more coils can be energized simultaneously. This design can be used for all
multicoil topologies by the Qi specification.
Coils Used for Wireless Charging Application Note, Rev. 0, 01/2014
Freescale Semiconductor, Inc.
5
Figure 6 shows the next possible topology for the multicoil system.
Figure 6 Independent bridge drivers
This topology uses several bridge drivers to energize each coil. The half or full-bridges are controlled by
the wireless charging controller. It is possible to energize one or more coils simultaneously. The selection
of the final topology depends on the transferring power level and finally on the cost. If a design requires
the EMI filter and small frequency range (automotive sector), the multiplexer can be a better selection.
5 Conclusion
The use of the WCT1001A or other controllers from the WCTxxxx family, together with the binary
wireless charging library considerably simplifies the whole design process and provides fully tested
functions for a reliable application.
6 References
The following reference is available on freescale.com:
Wireless Charging
7 Revision history
6
Revision number
Date
Substantial changes
0
01/2014
Initial release
Coils Used for Wireless Charging Application Note, Rev. 0, 01/2014
Freescale Semiconductor, Inc.
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©2014 Freescale Semiconductor, Inc.
Document Number: AN4866
Rev. 0
01/2014